1. Field of the Invention
The present invention relates to the field of photolithography to form integrated circuits and more particularly to the field of photoresists used in photolithography.
2. Discussion of Related Art
Photolithography is used in the field of integrated circuit processing to form the patterns that will make up the features of an integrated circuit. A photoresist is employed as a sacrificial layer to transfer a pattern to the underlying substrate. This pattern may be used as a template for etching or implanting the substrate. Patterns are typically created in the photoresist by exposing the photoresist to radiation through a mask. The radiation may be visible light, ultraviolet light, deep ultraviolet light, and extreme ultraviolet (EUV) light, or an electron beam. In the case of a “direct write” electron beam, a mask is not necessary because the features may be drawn directly into the photoresist. Most photolithography is done using chemically amplified systems or nonchemically amplified systems (e.g. “i-line”). In the i-line method an additive in the photoresist becomes soluble in developer when irradiated and the additive also renders surrounding species soluble and thus the exposed photoresist may be removed by a developer. In the chemical amplification (CA) method the radiation applied to the photoresist causes the decomposition of a photo-acid generator (PAG) that causes the generation of a small amount of acid catalyst throughout the exposed resist. The acid in turn causes a cascade of chemical reactions either instantly or in a post-exposure bake that increase the solubility of the resist such that the resist may be removed by a developer. An advantage of using the CA method is that the chemical reactions are catalytic and therefore the acid is regenerated afterwards and may be reused, thereby decreasing the amount of radiation required for pattern formation in the resist and thus enabling the use of shorter wavelengths of light such as EUV that are produced by weaker light sources. The photoresist may be positive tone or negative tone. In a positive tone photoresist the area exposed to the radiation will define the area where the photoresist will be removed. In a negative tone photoresist the area that is not exposed to the radiation will define the area where the photoresist will be removed. The CA method may be used with either a positive tone photoresist or a negative tone photoresist.
As the scale of the dimensions of the structures formed by etching materials masked by photoresist materials are scaled down, the performance of the photoresist materials must increase. For one thing, critical dimension (CD) control must be increased. As dimensions of the structures are scaled down, the amount of permissible error in the critical dimensions of the structures decreases. Also, the line width roughness of the areas etched must be minimal to accommodate for smaller dimensions and improved device performance. The defectivity of the photoresists must also be minimized. Similarly, collapse of the photoresist must be minimized. Defectivity and collapse are believed to occur, in part, when the mixture of the photoresist components are not uniformly distributed which results in uneven performance within the photoresist and thus the photoresist may not be properly patterned. Each of these issues poses challenges to chemically amplified (CA) resists.
To deal with these issues in the past, the choice of photoacid generator (PAG), as well as control of polymer molecular weight, polymer primary structure, molecular weight distribution, polymer side groups's structure and as well as quencher and other additives' structures and these species' relative ratios, in addition to the solvent system, are used to modulate the resulting performance of the photoresist formulation. Many of these approaches address the issue of preventing the PAG from diffusing beyond the region irradiated to reduce line width roughness and improve CD control. Approaches that address controlling the diffusion of the PAG include using a bulky PAG such as triphenylsulfonium perfluorooctanyl sulfonate (PFOS) to create upon photolysis at the appropriate wavelength a bulky PAG that will diffuse only a short length. The minimization of the diffusion of the PAG has also been taken one step further by attaching the PAG to a side chain of the photoimageable species, such as a polymer. Preventing the diffusion of the photoacid has also been approached by attaching the quencher to the photoimageable species. The drawback to these approaches is that the components within the photoresist are still not evenly distributed and the resulting system's constituent species are not all the same size. The lack of even distribution and uniformity in the size of the components may cause the photoacid to diffuse too much or too little before it is scavenged by a quencher. This may reduce the photospeed of the photoresist, and cause line roughness and loss of CD control.